The present invention relates to a fluorescent X-ray film thickness gauge, and more particularly to an improvement of irradiating an X-ray to a sample accurately and of improving detection efficiency by coinciding an X-ray axis with an optical axis of a viewing means.
In the conventional fluorescent X-ray film thickness gauge of this type, when a measuring point "P" on a sample 1 is irradiated by an incident X-ray beam 3 from an X-ray tube 2 positioned above the sample 1 as shown in a principle diagram in FIG. 1, a fluorescent X-ray 4 is emitted from the measuring point "P" to a detector 5 which measures the amount of fluorescent X-rays. On this occasion, the point to irradiate an X-ray is determined by viewing the sample 1 by a microscope and projector as viewing means 6 positioned above the sample. However, since an optical axis 7 of the viewing means 6 does not coincide with the X-ray axis 3 of the X-ray tube 2, i.e. since the surface of the sample 1 is not irradiated at a right angle but at an inclined angle against the X-ray axis 3, the shape of the irradiating point of the X-ray on the sample 1 is not circular but elliptic as indicated by a dotted line in FIG. 2. For instance, when the plating film thickness of very small plating areas provided on an exceedingly precise member such as an IC circuit is measured, the shape of the irradiating point is elliptic even if the X-ray beam is thinned, and the neighboring portions are also irradiated unnecessarily. As a result, the film thickness cannot be measured accurately. Further, if the sample 1 travels vertically, the shape of the irradiating point becomes elliptic and the irradiating position moves, so the measuring accuracy is exceedingly bad condition.
Accordingly, it is an object of the present invention to provide an exceedingly effective method of eliminating the abovementioned drawbacks and make an accurate positioning of X-ray irradiating point by coinciding an X-ray axis from an X-ray tube with an optical axis of a viewing means.